1. Field of the Invention
This invention relates generally to high temperature set retarded well cement compositions and methods, and more particularly, to set retarded cement compositions which are substantially non-thinning at high temperatures and methods of cementing with such compositions.
2. Description of the Prior Art
In cementing operations carried out in oil, gas geothermal and water wells, a hydraulic cement is normally mixed with sufficient water to form a pumpable slurry, and the slurry is pumped into a subterranean zone to be cemented by way of a well bore penetrating the zone. After placement, the cement slurry sets into a hard mass.
In primary cementing whereby casing and/or liners are bonded within the well bore to the subterranean zones penetrated thereby, cementing is accomplished by introducing a cement composition into the annular space between the casing or liner and the well bore. Generally, the cement composition is pumped downwardly within the casing or liner to the bottom thereof and then upwardly into the annulus.
One or more of the zones adjacent the cemented annulus can, and often do, contain fluids under pressure. In order to prevent such pressurized fluids from entering and flowing through the cement composition containing annulus, the cement composition density is increased by including a weighting material such as hematite therein. Fluid loss reducing and other additives are also commonly used in well cement compositions.
Another problems often encountered in well cementing relates to the rate at which the cement composition develops gel and compressive strengths after being placed. In order to allow enough time to mix the cement composition on the surface and then pump it down the well bore and into a desired location therein, a variety of set retarding additives have been utilized. While set retarding additives successfully extend the pumping time between mixing and setting of cement compositions, they are temperature sensitive, i.e., the higher the temperature which a cement composition including such an additive will be exposed to, the greater the quantity of additive required. A problem which arises in the cementing of deep high temperature wells wherein the cement composition must contain relatively high quantities of set retarding additive is that excessive thinning of the cement composition often takes place. Such thinning leads to the settling of solids in the cement composition and the separation of free water therefrom which can bring about undesirable consequences, e.g., bridging, water pockets within the cement column, poor bonding, etc. In deviated wells and geothermal wells, free water can be responsible for fluid migration to other formations, gas leakage and collapsed pipe.
Prior attempts to solve high temperature thinning of set retarded cement compositions have included combining conventional viscosifying agents such as substituted celluloses or bentonite with the compositions. Generally, however, the use of such viscosifying agents results in high surface viscosities which in turn makes mixing and pumping difficult, time consuming and expensive.
By the present invention, improved set retarded well cement compositions for cementing deep hot wells and methods of using such compositions are provided. The set retarded cement compositions are substantially non-thinning at high temperatures while having normal surface viscosities and other properties.